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In the combiner, hit the + icon to dynamically create a light filter. Use the x icon to remove it. This combiner groups all the light filters under specific combining operators. That is, all the light filters under "Mult" will be multiplying by each others while "Max" will take the maximum of the light filters in the max group.
You cannot wire a Pxr light filter combiner into another Pxr light filter combiner.